The copending U.S. patent of Coyle et al., U.S. Pat. No. 4,859,827, granted Aug. 22, 1989, is an example of the literature describing laser welding of various metal workpieces. The Coyle et al. patent describes a method for laser welding a sleeve to an optical fiber ferrule. The optical fiber can be rotationally aligned within the sleeve, and, after alignment has been obtained, laser welding can conveniently be used to bond the sleeve to the ferrule so as to insure permanent rotational alignment of the fiber with respect to the sleeve.
Especially when optical fibers are to be used in remote environments such as undersea cables, they must be made with processes that insure long-term reliability. The optical fiber ferrules and sleeves, as described in the Coyle patent, are typically made of a material such as Kovar (a trademark for an iron-nickel-cobalt alloy) to minimize the effects of differential thermal expansion. The Kovar in turn is typically plated with a thin protective layer of gold.
We have found that laser welds of gold-plated Kovar are susceptible to microcracking, which can seriously impair their reliability. Our study of the problem has shown that this microcracking is caused by an excess of gold in the Kovar weld. Thus, one way of avoiding the problem would be to etch away a portion of the gold plating prior to making the weld. This of course would significantly increase the cost and complexity of Kovar welds and would complicate the mass-production of components containing aligned optical fibers. Accordingly, there is a need for a method for making reliable welds in gold-plated Kovar which is simple, inexpensive, and which requires little operator skill.